soaking in a bucket of water restores flexibility so i assume the reused ones were soaked.

an 18" dia bundle when tied tight was quite a bit fatter before the roll, squash, shuffle and secure. maybe the professionals have a better method than i but i would need around 12 ft to get that size nice and tight.

iirc a lot of the london fuel was from pollard rather than coppice perhaps that makes a difference to the types of twigs available for ties? epping forest has some ace ancient pollard some of which has been reworked after a considerable time unharvested.
as well as fuel tree hay ( leaves and green twigs ) was collected for animal fodder for the working equines and milk cows of the city which again might make for a lack of long withies?

The usual way to make faggots or bavins as they are also called, is to gather all the sticks into a frame, put a chain or chains round it with a stirrup on one end and pull tight, then tie. I have seen it done and use a similar method to clamp my besom heads. Yes, soaking will restore flexibility, but still think after a while the withies would get rather fragile. They would need to be selected, but would come from cutting of certain stools I think and might be a crop in their own right, not a general by-product of something else.

In the New Forest holly is cut high, but not high enough to be a true pollard, to feed animals in winter. Result is that start up a chainsaw there and you are surrounded by ponies waiting for food.

The New Forest ponies have it down to a fine art WW. Several of them hang around in the road for an inexperienced (in their ways) motorist to come along, then one or two others beg at the windows. If you know their ways you just edge gently forward and they have to give way, so go away grumbling.

just by chance i saw a woodcut drawing yesterday. the bloke was tying a faggot using a withy. he was holding it down with his foot and using the withy to help roll and lever it tight.

it looked like twice round and back through the bight with the trunk heel end of the withy was common enough to get recorded by the artist. as soon as i saw it i thought "that is how i do it " which was a bit spooky.

a bit of a strange coincidence ( or i miss a lot of "irrelevant" woodcuts of everyday tasks )

if using this method the size of the finished faggot is limited by the length of the available withies and the loose to tight volume ratio of the twigs being packed.

at a guess beech and oak pollard twigs ( eg epping ) would start as a pretty big bunch to give a decent weight of tightly bound faggot ready for transport to a baker.

I think there were standard sizes of faggot, and the size would also be dictated by the size of the oven. I seem to recall there being something in Silva about it, but can't remember what the sizes were.